Own posts in ap_timeline have author.url set to the publication URL
(site root like "https://rmendes.net/") with no /@handle or /users/handle
pattern. extractUsername("/") returns "" which falls back to "unknown".
Fix: set module-level local identity (publicationUrl + handle) at plugin
init via setLocalIdentity(). serializeStatus() compares item.author.url
against the publication URL and passes isLocal:true + handle to
serializeAccount() when they match.
This is zero-cost for callers — no signature changes needed at the 20+
serializeStatus() call sites.